The UN nuclear watchdog says Iran has stopped a team of inspectors from visiting a key military site.
The International Atomic Energy Agency (IAEA) says no deal was been reached on inspecting the Parchin site, south of Tehran, despite "intensive efforts".
The inspectors had sought to clarify the "possible military dimensions" of Iran's nuclear programme.
Iran says its programme is for peaceful purposes, but the West suspects it is geared towards making weapons.
The IAEA says its team is returning from Iran without a deal after two days of talks. The first round of discussions in January also failed to produce a result.
"It is disappointing that Iran did not accept our request to visit Parchin," IAEA Director General Yukiya Amano said in a statement.
It is suspected that Parchin was the site where explosives related to nuclear weapons may have been tested
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